R. S. Williams — Crustacean from the Devonian. 49 



Above are given all the characters of which the specimen 

 presents any reasonable suggestion. I have ventured to put 

 an interpretation upon some of the characters for which the 

 evidence is slight in the hope that those possessing specimens 

 of any kindred forms may throw light upon this one by con- 

 firming the interpretation here given or suggesting a better one. 



Fig. 1 represents very fairly the actual appearance of the 

 specimen in size and details. It is a photo-engraving from a 

 drawing of the original and photographs of it made by Pro- 

 fessor E. C. Cleves of Cornell University. 



Dimensions — Total length 10* cm 



Greatest width 5*7 



Length of telson (about) 4- 



Length of buckler (about) 2- 



Greatest thickness of telson 0*7 



Horizon — Chemung Group, Upper Devonian ; the " third oil 



sand " of I. C. White, 2d Pa. Survey. 

 Locality— LeBoeuf, Erie County, Pennsylvania. 



The original specimen is among the collections of the U. S. 

 Geological Survey, and will be deposited ultimately in the 

 National Museum. 



Comments. — This specimen throws back the known range of 

 Prestwichia, or at least the type to which this genus belongs, 

 to an earlier stage than heretofore reported. The earliest pre- 

 viously known Prestwichia occurs in the Carboniferous. 



If my interpretation of its characters be correct, Prestwichia 

 bears closer relations to Limulus than is suggested by other 

 known specimens, and also it possesses features linking it with 

 Trilobites and Eurypterids. 



Explanation of Figures. 



Fig. 1. Prestwichia Eriensis "Williams, sp. n. A sandstone cast representing the 



under surface ; natural size. 

 Fig. 2. Diagram of the same. 



A. Cephalic shield. 



B. ? Hypostoma. 



C. Genal spines of the cephalic shield. 



D. Thoracico-abdominal buckler. 



E. Telson. 



F. First (and second ?) segments of thorax (? anchylosed). 

 G-<7. Marginal spiues of the buckler. 



H. Flat median depression extending across the buckler and upon the 



telson. 

 ii. Longitudinal ridges of the buckler. 

 KK. Portions of the gnathopods. 

 L. ? Foliaceous terminations of the last gnathopods. 

 M. Position of the mouth. 

 N. Probable place of articulation of the telson. 

 Fig. 3. Theoretical diagram of upper side. 

 Cornell University, April, 1885. 



Am. Jour. Sci. — Third Series, Vol. XXX, No. 175, July, 1885. 

 4 



